1. Field of the Invention
The present invention relates to an impeller of a centrifugal fan, and more particularly to an improvement of a multi-blade impeller for use in a noiseless centrifugal fan.
2. Description of the Prior Art
The detailed construction of the conventional centrifugal fan has been publicly known. It has been known that if the number of blade of the impeller increases the generation of eddy current is reduced, so that the noise due to the rotation of the impeller is reduced. Especially, a small size centrifugal fan using a multi-blade impeller having ten or more blades is practically used in a point of view of the generation of eddy current.
FIG. 21 is a perspective view of a conventional multi-blade impeller 1 and FIG. 22 is a vertically sectional side view thereof.
A reference numeral 2 denotes a blade of the impeller 1, 1-1 denotes an outer peripheral surface of the impeller 1, 1-3 an inner peripheral surface of the impeller 1, 1-4 an inlet side end surface and 1-5 a blade holding base side end surface.
The present invention contemplates an impeller of 250 mm in diameter having more than fifty blades. In said conventional impeller 1, the outer and inner diameters of the inlet side end surface 1-4 are the same with that of the base side end surface 1-5, respectively.
In said conventional construction, the eddy current suppression effect can be obtained because a number of blades are used. However, there arises such a problem to be solved that an air current of high speed comes into an air current of low speed at the outer peripheral surface 1-1 of the impeller 1 because the current speed is increased gradually from the inlet side end surface 1-4 to the base side end surface 1-5, so that a further eddy current is generated.
Further, in said conventional construction, at a corner portion formed between the inner peripheral surface 1-3 and the inlet side end surface 1-4, such a phenomenon that an air current is braked away from the blades as shown in FIG. 23 is presented, so that an air current flowing into the blade is disturbed and the noise is generated.
As stated above, if the number of blade is increased the generation of the eddy current becomes small and the noise due to the rotation can be suppressed, but the distance between the adjacent blades becomes small, so that the inlet angle .beta.1 of the blade at the inner peripheral surface of the impeller must be set to 90.degree. and the outlet angle .beta.2 of the blade at the outer peripheral surface of the impeller must be set to 90.degree. according to the manufacturing requirement.
FIG. 24 shows a vertically sectional side view of a conventional centrifugal fan 6 having a multi-blade impeller 1 mounted rotatably in a casing 7. FIG. 25 shows a conventional multi-blade impeller 1 having fifty blades 2 of equal length arranged radially each extending from an inner peripheral edge 8-1 to an outer peripheral edge 8-2. A cross sectional area of a current path 9 relating to an air quantity is practically determined by a distance between adjacent blades in the vicinity of the inner peripheral edge 8-1, and it is well known that the air quantity becomes small if the blade number is increased.
If the blades are arranged radially, and the inlet angle .beta.1 of the blade at the inner peripheral surface of the impeller is set to 90.degree. and the outlet angle .beta.2 of the blade at the outer peripheral surface of the impeller is set to 90.degree. at the inlet side, such a phenomenon that an air current is braked away from the blades is presented, so that the noise is generated.
Further, at the outlet side, the eddy current is generated easily when the air is flowed, thereby causing the noise to be generated.
Further, in order to prevent the air quantity from being reduced, the blade number is reduced at the sacrifice of the reduction of noise, or the blade thickness is reduced in consideration of the durability of the blade in operation.